Nonquiescent Relaxation in Entangled Polymer Liquids After Step Shear

Large step shear experiments revealed through particle tracking velocimetry that entangled polymeric liquids display either internal macroscopic movements upon shear cessation or rupturelike behavior during shear. Visible inhomogeneous motions were detected in five samples with the number of entanglements per chain ranging from 20 to 130 at amplitudes of step strain as low as 135%

Dislocation-Controlled Perforated Layer Phase in a Peo-B-Ps Diblock Copolymer

Small angle x-ray analyses show that the shear-induced hexagonal perforated layer phase in a poly(ethylene oxide)-b-polystyrene diblock copolymer consists of trigonal (R3(overbar)m) twins and a hexagonal (P6(3)/mmc) structure, with trigonal twins being majority components. Transmission electron microscopy reveals that the hexagonal structure is generated through sequential intrinsic stacking faults on the second layer from a previous edge dislocation line, while the trigonal twins are formed by successive intrinsic stacking faults on neighboring layers due to the plastic deformation under mechanical shear

Onset of Tethered Chain Overcrowding

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature T-x increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density (σ) over tilde of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at (σ) over tilde*similar to3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and T-x. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks

Sequence Analysis of Styrenic Copolymers by Tandem Mass Spectrometry

Styrene and smaller molar amounts of either m-dimethylsilylstyrene (m-DMSS) or p-dimethylsilylstyrene (p-DMSS) were copolymerized under living anionic polymerization conditions, and the compositions, architectures, and sequences of the resulting copolymers were characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and tandem mass spectrometry (MS2). MS analysis revealed that linear copolymer chains containing phenyl–Si(CH3)2H pendants were the major product for both DMSS comonomers. In addition, two-armed architectures with phenyl–Si(CH3)2–benzyl branches were detected as minor products. The comonomer sequence in the linear chains was established by MS2 experiments on lithiated oligomers, based on the DMSS content of fragments generated by backbone C–C bond scissions and with the help of reference MS2 spectra obtained from a polystyrene homopolymer and polystyrene end-capped with a p-DMSS block. The MS2 data provided conclusive evidence that copolymerization of styrene/DMSS mixtures leads to chains with a rather random distribution of the silylated comonomer when m-DMSS is used, but to chains with tapered block structures, with the silylated units near the initiator, when p-DMSS is used. Hence, MS2 fragmentation patterns permit not only differentiation of the sequences generated in the synthesis, but also the determination of specific comonomer locations along the polymer chain

Detection of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution

A fluorescence method is described for the measurement of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution. The method requires two samples of the diblock copolymer. One sample is labelled with a Förster donor, the other sample is labelled with a Förster acceptor. Successful application of the method is demonstrated with diblock copolymers composed of polystyrene and poly(ethylene oxide). The donor and acceptor are naphthalene and pyrene, respectively. The label is covalently attached to the copolymers at the junction points between the two blocks. Solutions with micelles are formed independently by the two labelled samples. At the time of mixing of the two solutions, no micelle contains both a donor and an acceptor. Micelles containing both types of labels may be formed at later times as a consequence of the exchange of labelled chains. The efficiency of nonradiative singlet energy transfer from naphthalene to pyrene is measured as a function of time after mixing of the two solutions. At 60° C the rate constant deduced from the time dependence of the fluorescence is on the order 10-5 s-1. At ambient temperature, however, no exchange can be detected, presumably because of the difficulty in extraction of a polystyrene block from the glassy core. © 1992 Springer-Verlag